Phosphonates are compounds which are used in water treatment systems to control scale formation and corrosion. The concentration of phosphonates in the water must be maintained within specific limits in order to optimize and maintain the water treatment system's performance. Too little phosphonate in the water will lead to scale formation, while too much phosphonate may cause corrosion. Using more phosphonate than needed to prevent scale formation also is a waste of phosphonate.
Currently, the concentration of phosphonate present in the water of a water treatment system is typically measured in one of two ways. The most conventional method relies on the analysis of grab samples. That is, a sample of water is withdrawn from the system and analyzed by conventional manual techniques. For example, the phosphonate in the water sample is oxidized to phosphate using UV light and a chemical oxidizing agent such as potassium persulfate. The concentration of phosphate then is measured colorometrically, and correlated to the concentration of phosphonate present in the original sample. See Blystone et al., Internat'l Water Conference, Pittsburgh, Pa. (1981). As used herein, the term "grab sample method" refers to the above-described method wherein a sample of water is withdrawn from the system and a chemical oxidizing agent is used in a wet bench laboratory analysis of the phosphate/phosphonate content. Grab sample methods are labor intensive, subject to human error, and take about 15 minutes for each grab sample. They cannot be automated readily because potassium persulfate is hazardous, not stable, and would have to be replaced as it is degraded.
Another method for determining the concentration of phosphonate present in the water of a water treatment system uses inert tracers. In this method, an inert tracer that can be easily measured spectroscopically, such as a fluorescent compound, is added to the system water in a fixed ratio to the amount of phosphonate added to the system water. The level of tracer in the system water is measured from time to time and correlated to the concentration of phosphonate present in the system water. U.S. Pat. No. 4,783,314 describes an example of this type of method. This tracer method suffers from several drawbacks. First, this method measures only the concentration of tracer in the system water, and assumes that the ratio of tracer to phosphonate remains constant throughout the system. Because phosphonate is an active ingredient (and may decompose, precipitate, or be adsorbed), whereas the tracer is inert, this assumption may not be a valid one. Second, because the level of phosphonate itself is not determined, errors resulting from a failure of this assumption are not detected. Phosphonates cannot be measured directly by this method because they are not easily detectable spectroscopically.
Accordingly, there is a need for a method for determining the concentration of phosphonate present in an aqueous solution that is simple, accurate, and subject to automation.